Cadmium removal from aqueous solution by brown seaweed, Sargassum angustifolium

Abstract

Four kinds of indigenous seaweed were employed for assessing their soluble cadmium biosorption performance. Sargassum angustifolium revealed the greatest capacity in the range of equilibrium cadmium concentration lower than 0.5mmol l⁻¹. It was further examined by optimization, equilibrium, kinetic and thermodynamic studies. It was found that1 g l⁻¹ biosorbent at initial pH of 6 and 38 °C revealed the highest Cd²⁺ uptake. Kinetic studies revealed that the Cd²⁺ biosorption included a two-stage mechanism with an initial rapid stage during the first 30 min where ion exchange was the dominant mechanism. The process gradually reached equilibrium after 40-50 min of contact where the metal adsorption occurred too low due to the intraparticle diffusion. However, it was not the sole rate-limiting step. The pseudo-second order kinetic model, unlike the pseudo-first order, excellently described the experimental data in the whole range of contact time. The Langmuir isotherm model was more successful in describing the equilibrium data than the Freundlich and D-R models. Using this isotherm model, a relationship was proposed to predict the dose of biosorbent needed for removing specific initial cadmium concentration from aqueous solution or to meet a desire equilibrium cadmium concentration. The spontaneity and endothermicity as well as increasing randomness at the solid/solution interface during the biosorption were revealed by means of the thermodynamic studies.